hostlist parse fix

If a hostlist expression contained a separator after two open
brackets and one close bracket, this resulted in bad parsing.
Before:
$ scontrol show hostnames a[1-2]b[1,2]
a[1-2]b[1]
2]
After:
$ scontrol show hostnames a[1-2]b[1,2]
a1b1
a1b2
a2b1
a2b2